The present invention relates to a method for preparing thin, polycrystalline metal oxide films. The films prepared by the method of the present invention are dense (i.e., substantially free of cracks and pinholes) and may be used, for example, as an electrolyte or electrode in intermediate temperature solid oxide fuel cells (SOFCs) or as gas separation membranes.
A fuel cell is a static device that converts the chemical energy in a fuel directly, isothermally, and continuously into electrical energy. Fuel and oxidant (typically oxygen in air) are fed to the cell in which an electrochemical reaction takes place that oxidizes the fuel, reduces the oxidant, and releases energy. The energy released is in both electrical and thermal forms, the electrical part providing the required output. In a typical power generation plant employing fuel cells, hydrocarbon fuel (e.g., natural gas) or gasified coal is reformed first to produce hydrogen-rich and sulphur-free gas that enters the fuel cell stack where it is electrochemically "burned" to produce the electrical and thermal outputs. The electrical output of a fuel cell is low-voltage high-current dc. By utilizing a properly organized stack of cells and an inverter, utility-grade ac output is obtained.
Fuel cells generating electricity from natural gas offer significant advantages over conventional power generation systems including improved reliability and safety and reduced airborne emissions. Also, since a fuel cell completely bypasses the thermal-to-mechanical conversion involved in a conventional power plant and since its operation is isothermal, fuel cells are not Carnot-limited. Efficiencies in the range of 43 to 55% are forecasted for modular dispersed generators featuring fuel cells. The possibility of using fuel cells in combined heat and power units provides the cleanest and most efficient energy system option utilizing valuable natural gas resources.
One of the most promising types of fuel cells being developed are SOFCs. SOFCs are particularly desirable as alternatives to conventional power sources due to their reliability, increased power-to-weight and power-to-volume ratios, simplicity, and environmental advantage over other types of fuel cells. These characteristics make SOFCs ideal for use in remote electrical power generation applications such as in space stations and satellites. Alternatively, SOFCs can be integrated with a coal gasifier and a steam bottoming cycle to form a more conventional electrical power generation system. The reliability of SOFCs is mainly attributed to stability of the components as well as the presence of low kinetic barriers at the electrode/electrolyte interfaces.
SOFCs are typically of planar or tubular construction and comprise layers or films of various polycrystalline metal oxides which form the electrolyte and electrode components of the fuel cell. The electrolyte of choice in state-of-the-art SOFCs is a film made from (ZrO.sub.2).sub.1-x (YO.sub.1.5).sub.x (YSZ), while suitable electrodes are comprised of films made from La.sub.1-x Sr.sub.x MnO.sub.3 (LSM) or La.sub.1-x Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3 ( LSCF ). In the preceding formulas, x and y have values between 0 and 1 and can be varied to provide an oxide of the desired nominal composition.
Conventional SOFCs typically comprise a layer of YSZ electrolyte 40-160 .mu.m thick and require operating temperatures of approximately 1000.degree. C. to achieve adequate oxygen transport across the electrolyte. Conventional SOFC fabrication techniques require even higher temperatures. Reduction in fuel cell operating and fabrication temperatures will improve cell performance not only due to reduced interfacial resistance between the electrolyte and the cathode, but also due to a reduction in other related problems such as thermal stresses, interdiffusion, sealings and interconnections.
Reduced operating temperatures can be achieved by developing methods of preparing thinner metal oxide films for use as electrolytes and electrodes in SOFCs. A thinner electrolyte, for example, will provide a shorter path for ion transfer and will result in the electrolyte exhibiting less ohmic resistance at reduced temperatures (e.g., 600.degree.-800.degree. C.) as compared to conventional, thicker electrolytes.
Metal oxide films are also used as gas separation membranes. For example, a gas separation membrane comprised of a metal oxide film may be incorporated in an air pump used to separate pure oxygen from air. In such a device, the oxide film serves as a membrane which selectively transfers the desired component of the air mixture (i.e., oxygen) across the membrane.
Various attempts have been made to fabricate thin metal oxide films using electrochemical vapor deposition (EVD), plasma spraying, RF sputtering, spray pyrolysis, and sol-gel methods, etc. Among these various alternative methods, the sol-gel derived oxide films possess improved homogeneity, higher purity, and offer the advantage of processing a wide range of oxide compositions at somewhat lower temperatures as compared to the other synthesis techniques. However, sol-gel alkoxide precursors are moisture sensitive and their shelf-life is relatively short. Accordingly, a need exists for a film preparation method capable of providing high quality, thin, dense, polycrystalline metal oxide films at even lower processing temperatures. Dense films are particularly important in SOFC and gas separation membrane applications. On the other hand, lower processing temperatures are desirable because they decrease unwanted thermal interactions and interdiffusion between the film and the substrate on which the film is deposited as well as reduce the tendency for cracks and other discontinuities to form in the film.